The-present invention relates to a liquid crystal display comprising a liquid crystal cell, a linearly polarizing membrane, a cholesteric liquid crystal layer and a quarter wave plate. The cholesteric liquid crystal layer is divided into two or more areas having different spiral pitches of cholesteric liquid crystal.
Liquid crystal displays include various display modes, such as TN (twisted nematic), IPS (in plane switching), FLC (ferroelectric liquid crystal), OCB (optically compensatory bend), STN (super twisted nematic), VA (vertically aligned), HAN (hybrid aligned nematic) and GH (guest host). A liquid crystal display of any mode essentially comprises a liquid crystal cell and a linearly polarizing membrane. Rod-like liquid crystal molecules (which are aligned depending on whether voltage is applied or not) in the liquid crystal cell optically affect on polarized light having passed through the linearly polarizing membrane to display an image.
A method using a color filter is practically adopted in a liquid crystal display to display a color image, though various methods have been proposed. The color filter for liquid crystal display is divided into two or more areas, which absorb light in different wavelength regions (usually, red (R), green (G) and blue (B) regions). Light is optically separated through the color filter into R, G and B components, and then the components are mixed to display the color image.
Dyes or pigments have been conventionally used to produce the color filter.
Japanese Patent Provisional Publication Nos. 8(1996)-234196, 8(1996)-297280, 10(1998)-177167 and lO(1998)-260387 propose a color filter comprising a cholesteric liquid crystal layer formed from cholesteric liquid crystal in place of the conventional filter of the dyes or pigments. Since the cholesteric liquid crystal layer gives a selective reflection spectrum having nearly ideal square shape, the layer does not lower the purity of displayed color. The spiral pitch of cholesteric liquid crystal in the layer can be adjusted according to the wavelength of R, G or B.
The cholesteric liquid crystal layer reflects a light component circularly polarized in the same direction as the spiral of the liquid crystal. In the case that the cholesteric liquid crystal layer is used as a color filter, a quarter wave plate, which converts linearly polarized light into circularly polarized light (or which converts circularly polarized light into linearly polarized light) should be used in combination with the cholesteric liquid crystal layer.
In a liquid crystal display comprising the cholesteric liquid crystal layer and the quarter wave plate, a performance of the display depends on functions of not only the cholesteric liquid crystal layer but also the quarter wave plate. Most of the conventional quarter wave plates give phase retardation of xcex/4 (quarter wave) at particular wavelengths. Accordingly, the quarter wave plate must be divided into plural areas corresponding to areas of the cholesteric liquid crystal layer, which absorb light in different wavelength regions (usually R, G and B).
Each of Japanese Patent Provisional Publication Nos. 5(1993)-27118, 5(1993)-27119, 10(1998)-68816 and 10(1998)-90521 discloses a phase retarder comprising two polymer films, which are so laminated that the phase retardation of xcex/4 is given within a wide wavelength region. However, the quarter wave plate comprising two polymer films is relatively thick. The thick plate offsets the most important advantage of the liquid crystal display (namely a very thin display device compared with the other displays).
An object of the invention is to provide a thin liquid crystal display comprising a cholesteric liquid crystal layer and a quarter wave plate, which is not divided into two or more areas.
The present invention provides a liquid crystal display which comprises a liquid crystal cell, a linearly polarizing membrane, a cholesteric liquid crystal layer and a quarter wave plate, said cholesteric liquid crystal layer being divided into two or more areas having different spiral pitches of cholesteric liquid crystal, wherein the quarter wave plate comprises an optically anisotropic layer A and an optically anisotropic layer B, one of said optically anisotropic layers A and B being formed from liquid crystal molecules, the other being formed from liquid crystal molecules or made of a polymer film, and said quarter wave plate giving a retardation value per wavelength within the range of 0.2 to 0.3 at each wavelength of 480 nm, 550 nm and 630 nm.
The applicants have studied a quarter wave plate, and have succeeded in preparing a quarter wave plate of wide wavelength region, which suitably works with a cholesteric liquid crystal layer. The quarter wave plate comprises two optically anisotropic layers A and B. At least one of them is formed from liquid crystal molecules. The quarter wave plate comprising the two optically anisotropic layers A and B can give xcex/4 within a wide wavelength region. When the quarter wave plate of the wide wavelength region is used with a cholesteric liquid crystal layer in a liquid crystal display, the quarter wave plate can convert linearly polarized light into circularly polarized light (or convert circularly polarized light into linearly polarized light) in a wide wavelength region without dividing the layer into two or more areas.
At least one of the two optically anisotropic layers A and B is formed from liquid crystal molecules. Accordingly, the quarter wave plate can be thinner than a conventional plate comprising laminated two polymer films. Therefore, the quarter wave plate of the present invention is thin and light enough to be suitably used in a liquid crystal display.
Further, optical characteristics of the optically anisotropic layer can easily be controlled where the layer is formed from liquid crystal molecules. For example, the optical direction of the optically anisotropic layer can easily be set by selecting a rubbing direction of the liquid crystal molecules. Accordingly, it is unnecessary to cut films into chips in a conventional manner.
As is described above, it is not necessary to divide the quarter wave plate according to the invention into two or more areas. Therefore, the present invention can provide a thin liquid crystal display.